The present invention relates generally to cellular telephone systems, and more particularly, to a cellular telephone system that employs spread spectrum transmission to provide additional services without degrading the existing voice communications service, while utilizing much of the present cellular telephone infrastructure.
The cellular telephone band was designed to carry a large number of two-way voice conversations to mobile users. In addition to two way voice communication, there is interest in using the cellular telephone band to provide vehicle location and messaging services, emergency SOS information, and vehicle anti-theft protection services, for example. However, current systems that are confined to narrow band voice channels of the cellular band produce location accuracies that are about two orders of magnitude lower than are required to provide these services, and these are generally not useful for most applications.
Previous nationwide vehicle location systems, disclosed in U.S. Pat. Nos. 4,359,733 and 4,740,792, for example, have not come to fruition to date because of the relatively high start up capital cost of using satellites. Other nationwide location systems are planned (TELETRAC and GEOSTAR), but they operate in frequency bands other than the cellular band and therefore require a very expensive special purpose infrastructure. The IBM-Motorola mobile data service currently operational in the New York, Chicago and Los Angeles areas also requires a special purpose infrastructure which results in high usage fees.
The use of spread spectrum communications in conjunction with the cellular voice channel has been investigated in recent years. One implementation proposed by Qualcomm, Inc. of San Diego, Calif., would partition the cellular band such that a 1 MHz frequency band would be dedicated to each of the transmit and receive bands for spread spectrum voice communication, while the balance of the two bands would continue to provide standard cellular service. As spread spectrum systems increased in usage, the spread spectrum portion of the bands would increase in size to match the need. It is apparent that, since this proposed system requires its own dedicated frequency subbands, that the Qualcomm implementation of spread spectrum communication would interfere with the operation of the standard cellular voice channels.
Accordingly, it is an objective of the present invention to employ technology that overlays the narrow band cellular telephone voice signals with wide band spread spectrum signals to provide for messaging and vehicle tracking capabilities without adversely affecting the quality or capacity of the voice channels.